I saw some fresh in morrisons yesterday and I bought some frozen from asda last week. It's in a kind of tub rather than a bag tho. (incase you were looking through the freezers for a bag)
It's a "new thing" (ie not in the book). As far as I can recall, it's 100 g of cooked rhubarb tolerated on PP days, on PV days you can have as much as you like. Great for errr transit issues (as is a glass of ICE COLD water first thing )